# Fused Angles for Body Orientation Representation Philipp Allgeuer and Sven Behnke Institute for Computer Science VI Autonomous Intelligent Systems University.

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Fused Angles for Body Orientation Representation Philipp Allgeuer and Sven Behnke Institute for Computer Science VI Autonomous Intelligent Systems University of Bonn

Nov 18, 2014Fused Angles for Body Orientation Representation2 Motivation What is a rotation representation? A parameterisation of the manifold of all rotations in three-dimensional Euclidean space Why do we need them? To perform calculations relating to rotations Existing rotation representations? Rotation matrices, quaternions, Euler angles, … Why develop a new representation? Desired for the analysis and control of balancing bodies in 3D (e.g. a biped robot)

Nov 18, 2014Fused Angles for Body Orientation Representation3 Problem Definition The problem: Find a representation that describes the state of balance in an intuitive and problem-relevant way, and yields information about the components of the rotation in the three major planes (xy, yz, xz) Orientation A rotation relative to a global fixed frame Relevant as an expression of attitude for balance Environment Fixed, z-axis points ‘up’ (i.e. opposite to gravity)

Nov 18, 2014Fused Angles for Body Orientation Representation4 Problem Definition The solution: Fused angles (and the intermediate tilt angles representation)

Nov 18, 2014Fused Angles for Body Orientation Representation5 Uses of Fused Angles to Date Attitude Estimator [1] [2] Internally based on the concept of fused angles for orientation resolution NimbRo ROS Soccer Package [4] [5] Intended for the NimbRo-OP humanoid robot Fused angles are used for state estimation and the walking control engine Matlab/Octave Rotations Library [6] Library for computations related to rotations in 3D (supports both fused angles and tilt angles)

Nov 18, 2014Fused Angles for Body Orientation Representation6 Existing Representations Rotation matrices Quaternions Euler angles Axis-angle Rotation vectors Vectorial parameterisations

Nov 18, 2014Fused Angles for Body Orientation Representation7 Containing set: Parameters:3 ⇒ Minimal Constraints:None Singularities:Gimbal lock at the limits of β Features: Splits rotation into a sequence of elemental rotations, numerically problematic near the singularities, computationally inefficient Intrinsic ZYX Euler Angles

Nov 18, 2014Fused Angles for Body Orientation Representation8 Intrinsic ZYX Euler Angles Relevant feature: Quantifies the amount of rotation about the x, y and z axes ≈ in the three major planes Problems: Proximity of both gimbal lock singularities to normal working ranges, high local sensitivity Requirement of an order of elemental rotations, leading to asymmetrical definitions of pitch/roll Unintuitive non-axisymmetric behaviour of the yaw angle due to the reliance on axis projection

Nov 18, 2014Fused Angles for Body Orientation Representation9 Tilt Angles Rotation G to B ψ= Fused yaw γ= Tilt axis angle α= Tilt angle

Nov 18, 2014Fused Angles for Body Orientation Representation10 Tilt Angles Features: Geometrically and mathematically very relevant Intuitive and axisymmetric definitions Drawbacks: γ parameter is unstable near the limits of α!

Nov 18, 2014Fused Angles for Body Orientation Representation11 Fused Angles Rotation G to B Pure tilt rotation! θ= Fused pitch φ= Fused roll h= Hemisphere

Nov 18, 2014Fused Angles for Body Orientation Representation12 Fused Angle Level Sets

Nov 18, 2014Fused Angles for Body Orientation Representation13 Fused Angle Level Sets

Nov 18, 2014Fused Angles for Body Orientation Representation14 Intersection of Level Sets

Nov 18, 2014Fused Angles for Body Orientation Representation15 Fused Angles Condition for validity: Sine sum criterion Set of all fused angles:

Nov 18, 2014Fused Angles for Body Orientation Representation16 Sine Sum Criterion

Nov 18, 2014Fused Angles for Body Orientation Representation17 Mathematical Definitions By analysis of the geometric definitions:

Nov 18, 2014Fused Angles for Body Orientation Representation18 Representation Conversions Fused angles ⇔ Tilt angles Surprisingly fundamental conversions Representations intricately linked Fused angles ⇔ Rotation matrices, quaternions Simple and robust conversions available Tilt angles ⇔ Rotation matrices, quaternions Robust and direct conversions available Simpler definition of fused yaw arises Refer to the paper

Nov 18, 2014Fused Angles for Body Orientation Representation19 Tilt axis angle γ has singularities at α = 0, π …but has increasingly little effect near α = 0 Fused yaw ψ has a singularity at α = π Unavoidable due to the minimality of (ψ,θ,φ) As ‘far away’ from the identity rotation as possible Define ψ = 0 on this null set Fused yaw and quaternions Properties

Nov 18, 2014Fused Angles for Body Orientation Representation20 Properties Inverse of a fused angles rotation Special case of zero fused yaw

Nov 18, 2014Fused Angles for Body Orientation Representation21 Matlab/Octave Rotations Library https://github.com/AIS-Bonn/matlab_octave_rotations_lib https://github.com/AIS-Bonn/matlab_octave_rotations_lib Thank you for your attention!

Nov 18, 2014Fused Angles for Body Orientation Representation22 References

Nov 18, 2014Fused Angles for Body Orientation Representation23 Containing set: Parameters:9 ⇒ Redundant Constraints:Orthogonality (determinant +1) Singularities:None Features: Trivially exposes the basis vectors, computationally efficient for many tasks, numerical handling is difficult Rotation Matrices

Nov 18, 2014Fused Angles for Body Orientation Representation24 Containing set: Parameters:4 ⇒ Redundant Constraints:Unit norm Singularities:None Features: Dual representation of almost every rotation, computationally efficient for many tasks, unit norm constraint must be numerically enforced Quaternions

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